In a typical wireless communication network, wireless devices, also known as wireless communication devices, mobile stations, stations (STA) and/or user equipments (UEs), communicate via a Radio Access Network (RAN) to one or more core networks. The RAN covers a geographical area which is divided into areas or cell areas, with each area or cell area being served by an access point e.g. a transmission point such as a Wi-Fi access point or a radio base station (RBS), which in some networks may also be denoted, for example, a “NodeB” or “eNodeB”. The area or cell area is a geographical area where radio coverage is provided by the access point. The access point communicates over an air interface operating on radio frequencies with the wireless device within range of the access point.
A Universal Mobile Telecommunications System (UMTS) is a third generation telecommunication network, which evolved from the second generation (2G) Global System for Mobile Communications (GSM). The UMTS terrestrial radio access network (UTRAN) is essentially a RAN using wideband code division multiple access (WCDMA) and/or High Speed Packet Access (HSPA) for user equipments. In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity. In some RANs, e.g. as in UMTS, several access points may be connected, e.g., by landlines or microwave, to a controller node, such as a radio network controller (RNC) or a base station controller (BSC), which supervises and coordinates various activities of the plural access points connected thereto. This type of connection is sometimes referred to as a backhaul connection. The RNCs are typically connected to one or more core networks.
Specifications for the Evolved Packet System (EPS) have been completed within the 3rd Generation Partnership Project (3GPP) and this work continues in the coming 3GPP releases. The EPS comprises the Evolved Universal Terrestrial Radio Access Network (E-UTRAN), also known as the Long Term Evolution (LTE) radio access network, and the Evolved Packet Core (EPC), also known as System Architecture Evolution (SAE) core network. E-UTRAN/LTE is a variant of a 3GPP radio access technology wherein the access points are directly connected to the EPC core network rather than to RNCs. In general, in E-UTRAN/LTE the functions of an RNC are distributed between the access points, e.g. eNodeBs in LTE, and the core network. As such, the Radio Access Network (RAN) of an EPS has an essentially “flat” architecture comprising access points connected directly to one or more core networks, i.e. they are not connected to RNCs. To compensate for that, the E-UTRAN specification defines a direct interface between the access points, this interface being denoted the X2 interface.
There exist today many coordination schemes to improve overall system efficiency of the wireless communication network by either increasing signal strength or decreasing interference level such as Coordinated Multi-point (CoMP). Typical coordination schemes in the purpose of increasing signal strength are for example, downlink joint transmission and uplink joint reception wherein coordinated beamforming increases the signal strength of one wireless device without introducing interference to other wireless devices. CoMP is used to send and receive data to and from a wireless device from several transmission points to ensure that an optimum performance is achieved. Typical coordination schemes in the purpose of decreasing the interference are for example dynamic point blanking and dynamic point power control, wherein the signal quality in terms of Signal to Interference plus Noise Ratio (SINR) will be improved by reducing the interference power.
The access points may be coordinating the transmissions by exchanging or informing one another over a backhaul connection between the access points. The term backhaul may be used to describe the entire wired part of the wireless communication network between the access points but may comprise wireless connections as well. Depending on a delay of the backhaul connection, also referred to as a backhaul delay, between the serving access point and the coordination access point, the coordination schemes may be further categorized as fast, relaxed and slow coordination. Coordination schemes such as, Enhanced Inter-Cell Interference Coordination (eICIC), Further Enhanced Inter-Cell Interference Coordination (FeICIC), and LTE dual connectivity, are aiming at cell level coordination and may be applied on a slow backhaul connectivity where no scheduling information needs to be exchanged and a coordination period could be longer than a couple of ten scheduling periods.
The objective of fast and relaxed backhaul coordination is to coordinate the receptions/transmissions individually at wireless devices level where a scheduling decision of each wireless device needs to be exchanged from serving cell to the coordinator cell. Upon the reception of the scheduling decision, the access point of the coordinator cell may perform different coordination algorithms together with the serving cell at the allocated resources to improve the channel quality for the wireless device. Such algorithms are for example, joint reception in the uplink e.g. Uplink (UL) CoMP, joint transmission or muting interferers in downlink e.g. Downlink (DL) CoMP. The difference between fast coordination where the backhaul delay is limited to one scheduling period, e.g. one Transmission Time Interval (TTI), and the relaxed coordination, is that the backhaul delay typically is longer than one TTI but shorter than 10 TTIs.
One problem with a relax backhaul coordination is caused by the transmission delay introduced on the interface between the access points. When the backhaul delay is longer than a processing time at the wireless device, i.e., the time from a grant/assignment is sent to the wireless device to the time of the transmission made from the wireless device, the transmission in the serving cell will be made earlier than or before the scheduling decision is received at the access node of the coordinator cell. Thus, no gain will be achieved with the coordination. For time critical services such as Voice over LTE (VoLTE) with periodic traffic characteristics, it has been proposed to preconfigure the same scheduling decision for both serving and coordinator cells. The serving cell will transmit when there is data according to the configured scheduling decision, whereas the coordinator is trying to decode the transmission according to the configuration without any knowledge if there is transmission. The solution is not efficient and thereby limits the performance of the wireless communication network.